I have not noticed myself, but it seems Windows Phone 7 lacks any way to search the calendar.
I am sure there are many others out there for whom this lack of functionality is driving them mad.
Liquid Daffodil has the answer however, and all for only $0.99. With Calendar Search you can search all your calendar, both in the past and future, and dredge up those long forgotten details.
Calendar Search has a free trial and can be found in Marketplace here.